# std::has_single_bit

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< cpp‎ | numeric

Defined in header `<bit>` |  |
template< class T > constexpr bool has_single_bit( T x ) noexcept; |  | (since C++20)

Checks if x is an integral power of two.

This overload participates in overload resolution only if `T` is an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type).

### Parameters

x | - | value of unsigned integer type

### Return value

true if x is an integral power of two; otherwise false.

### Notes

Prior to P1956R1, the proposed name for this function template was `ispow2`.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_int_pow2` | `202002L` | (C++20) | Integral power-of-2 operations

### Possible implementation

First version
    template<std::unsigned_integral T>
        requires !std::same_as<T, bool> && !std::same_as<T, char> &&
                 !std::same_as<T, char8_t> && !std::same_as<T, char16_t> &&
                 !std::same_as<T, char32_t> && !std::same_as<T, wchar_t>
    constexpr bool has_single_bit(T x) noexcept
    {
        return x && !(x & (x - 1));
    }
Second version
    template<std::unsigned_integral T>
        requires !std::same_as<T, bool> && !std::same_as<T, char> &&
                 !std::same_as<T, char8_t> && !std::same_as<T, char16_t> &&
                 !std::same_as<T, char32_t> && !std::same_as<T, wchar_t>
    constexpr bool has_single_bit(T x) noexcept
    {
        return std::popcount(x) == 1;
    }

### Example

Run this code

    #include <bit>
    #include <bitset>
    #include <cmath>
    #include <iostream>

    int main()
    {
        for (auto u{0u}; u != 0B1010; ++u)
        {
            std::cout << "u = " << u << " = " << std::bitset<4>(u);
            if (std::has_single_bit(u))
                std::cout << " = 2^" << std::log2(u) << " (is power of two)";
            std::cout << '\n';
        }
    }

Output:

    u = 0 = 0000
    u = 1 = 0001 = 2^0 (is power of two)
    u = 2 = 0010 = 2^1 (is power of two)
    u = 3 = 0011
    u = 4 = 0100 = 2^2 (is power of two)
    u = 5 = 0101
    u = 6 = 0110
    u = 7 = 0111
    u = 8 = 1000 = 2^3 (is power of two)
    u = 9 = 1001

### See also

popcount (C++20) | counts the number of 1 bits in an unsigned integer (function template)
count | returns the number of bits set to true (public member function of `std::bitset<N>`)
test | accesses specific bit (public member function of `std::bitset<N>`)

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